Test cfg-gated contracttrait override methods

[leighmcculloch]

What

Extend tests/attributes with regression coverage for three #[cfg] shapes on #[contractimpl(contracttrait)] override methods that no test exercised: stacked cfgs that are all true, the cfg(not(...)) form, and multiple mutually exclusive cfg-gated overrides of the same trait function.

Why

Review feedback on #1760 flagged cfg on trait override methods as an area with unresolved edge cases, and investigation found current main already handles every shape correctly since #1871 — but the multi-override path in derive_contractimpl_trait_default_fns_not_overridden had no test at all, and a deliberate mutation of that loop still passed the suite before this change.

Known limitations

N/A

SemVer Change

• Major (vX._._) - Breaking change to the public API.
• Minor (v_.Y._) - Additive change to the public API.
• Patch (v_._.Z) - No change to the public API.

[github-actions]

semver:patch — all changes are confined to tests/ and tests-expanded/, which are test-only crates and not published.

• cargo semver-checks reports "no semver update required" for all six published crates (soroban-sdk, soroban-sdk-macros, soroban-spec, soroban-spec-rust, soroban-token-sdk, soroban-ledger-snapshot).
• tests-expanded/ shows new defaulted trait methods added to AttributeTrait and new client methods, but AttributeTrait lives in the unpublished tests/attributes crate — no published API is affected.
• No Cargo.toml changes (no MSRV, feature, or dependency changes in any published crate).

Nothing was unclear about this classification.

This classification is advisory; the author and reviewer decide the release type.

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